1 Overview of the sewage treatment plant
The design scale of the first phase of the Jiaozuo City sewage treatment plant is 10×104m3/d, and the long-term design scale is 25×104m3/d. The first phase of the project was constructed with a mixed loan from the French government. All plant equipment is imported from France and adopts the improved Carrousel oxidation ditch process, which is divided into three parts: pretreatment, biochemical treatment, and sludge treatment. The effluent water quality complies with the secondary standard of the "Pollutant Discharge Standard for Urban Sewage Treatment Plants" (GB18918-2002), and the generated sludge is directly mechanically dehydrated without digestion. The specific process flow is shown in Figure 1.
2 Defects in operation
Jiaozuo City Sewage Treatment Plant was officially put into operation in February 2003. After more than three years of continuous adjustment of the process, the effluent BOD, COD, NH3 -N, SS, TP, etc. all meet the secondary standards in the national "Pollutant Discharge Standard for Urban Sewage Treatment Plants" (GB18918--2002), but at the same time some problems were exposed during operation.
2.1 The effect of anaerobic phosphorus removal is poor
The existing selection tank is designed as an anaerobic selection tank, but the capacity of the selection tank in this plant is only 960.4m3, and water is fed at full load. Calculated at 10×10m/d, the anaerobic residence time of the effluent from the aeration grit chamber in the selection pool is only 13.8 minutes. To meet the anaerobic conditions, the residence time of sewage in the selection pool is at least 1.5 to 2 hours. The current The residence time is far from meeting the reaction conditions, and in the current process, the activated sludge does not flow back to the selection tank, but directly flows back to the oxidation ditch. In this way, the selection pool only functions as a distribution well and cannot achieve effective phosphorus removal at all.
2.2 The oxidation ditch has serious mud accumulation
In December 2005, during the maintenance of the oxidation ditch, water discharge observation was conducted on the 1 and 2 oxidation ditches, and it was found that the mud accumulation at the bottom of the oxidation ditch was relatively serious. , there is mud accumulation in almost every ditch, especially in the middle of each ditch and in the large corridor of the outlet channel. The mud accumulation in some areas is about 2m thick (see Figure 2).